The present invention relates to clamping assemblies for injection molding apparatus, and, more particularly, to structures which ensure that the two portions of the injection molding mold remain together during the injection of molten plastic.
A typical injection molding machine features a fixed platen and a complementary movable platen. The two platens are designed to come together so as to form between them a multi-part, typically a two-part, mold. The two platens are pressed together through the use of a suitable driven ram, such as a hydraulic piston or toggle mechanism, and the like. With the two parts of the mold held firmly together, a molten plastic is injected under great pressure into the mold, thereby filling it. Once the plastic has cooled and hardened, the two parts of the mold are separated and the finished item is removed prior to repetition of the cycle.
Because of the high pressures under which the molten plastic is injected it is imperative that the two parts of the mold be very firmly and accurately held together. Thus, in order to prevent the escape of molten plastic and the attendant deterioration in the quality of the injection molded product, it is essential that the two parts remain accurately aligned with each other even when the two parts are pushed together using high pressure.
For many years, and to a large extent even today, the accurate alignment of the fixed and movable platens was assured through the use of four tiebars. In other words, the square or rectangular movable platen features openings, typically near each of its four corners. Each opening slidably accommodates a tiebar which is firmly attached to the fixed platen. The movable platen is thus slidably supported by the tiebars which assure the proper alignment of the two platens even when the two platens are forced together under great pressure.
While the tiebars are effective in preserving the alignment of the platens, they do so at great cost. Thus, the presence of the tie bars in the region of the mold significantly reduces the portion of the mold available for molding and, perhaps more importantly, considerably complicates the manual or automatic removal of the product as well as the changing of the molds.
Several attempts have been made to provide an injection molding clamping assembly which dispenses with the troublesome tiebars. One of these is disclosed in U.S. Pat. No. 5,354,196 to Ziv-Av et al.
U.S. Pat. No. 5,354,196 discloses a clamping assembly which includes a base to which the fixed platen is connected and a clamping structure which nests within the base and which supports the movable platen. The base and clamping structure are arranged so that when pressure is applied to force the two platens together the clamping structure absorbs the force, keeps the base from bending and ensures a tight fit between the two platens.
Another clamping assembly which does away with the tiebars was disclosed by Hemscheidt Maschinentechnik Schwerin GmbH & Co at the K-95 Exhibition in Dusseldorf, Germany on or about October, 1995. The Hemscheidt device can be constructed so that when the two platens are forced together under great pressure, causing the outward bending of the support posts, the portion of the support posts which supports the platens bends in the opposite sense, thereby tending to compensate for the outward bending of the support posts and tending to maintain the horizontal alignment of the two platens.
While the existing configurations represent great improvements over the tiebar-based approach, there remains a widely recognized need for, and it would be highly advantageous to have, a reliable and tiebar-less clamping assembly for injection molding apparatus which effectively maintains the horizontal alignment of the platens and which further is able to accurately slidably support the movable platen.